The present invention relates to a novel photoinitiator composition, to photohardenable compositions containing the novel photoinitiator compositions, to photosensitive materials employing them, and to methods for two and three dimensional imaging using the same.
More particularly, the present invention relates to a novel photooxidizable initiator system which comprises a fluorone initiator and a coinitiator which is capable of accepting an electron from the fluorone upon exposure of the composition to actinic radiation and which is sensitive to visible light and which provides high depth of cure in the photohardenable composition.
The visible light photoinitiators used most commonly in industry are sensitive to ultraviolet and blue light but do not exhibit high sensitivity to wavelengths greater than about 450 nm and, more particularly, greater than about 500 nm. Most of these visible initiators are ketocoumarins, titanocenes, and others. Unfortunately, these photoinitiators are not useful with the least expensive radiation sources and particularly lasers which emit at longer wavelengths. The photoinitiators are insensitive at these wavelengths, other photoinitiators such as Eosin, Rose Bengal and the like are too slow for commercial applications. As a consequence, there is a need for more efficient initiators which are sensitive to longer wavelengths.
Many, if not the vast majority of conventional photoinitiators, also suffer from another disadvantage, namely, a limited depth of cure. The depth of cure of these photoinitiators is limited by Beers law, that is, the photoinitiator at the surface of a composition absorbs the activating radiation and limits the amount of radiation which can penetrate to greater depths within the composition and produce a complete cure beyond the surface layer. One means of improving depth of cure is to design photoinitiators which bleach as they absorb light and initiate polymerization. In this manner, as the surface layers are exposed and cured they transmit more radiation to the underlying layers enabling radiation to reach greater depths and increasing the depth of cure.
One example of a visible light sensitive photoinitiator is the cyanine borates described in U.S. Pat. No. 4,895,880.
In photoinitiated polymerizable processes, initiators play an important role in radiation absorption and the electron transfer sensitization step. In the most common scheme, certain initiators absorb radiation and form an excited initiator I* which is capable of accepting an electron from a suitable coinitiator which functions as an electron donor D. Reduction of the excited initiator I* produces the donor cation radical D.sup.+ which may be transformed into a free radical species R which is capable of initiating free radical polymerization. Free radical polymerization according to this scheme is called photoreductive initiated polymerization as illustrated in equations (1)-(4). ##STR1## where N.sup.+ is a positive species, R.multidot. is a free radical, and M is the polymerizable monomer.
In another scheme, the excited initiator I* acts as an electron donor wherein an electron is given up by the excited initiator and transferred to a suitable coinitiator which functions as an electron acceptor A. Oxidation of I* produces the acceptor anion radical A.sup.- .multidot. which in turn produces the negative species N.sup.- and the free radical R.multidot. which is then capable of initiating free radical polymerization. Free radical polymerization according to this scheme is referred to as photooxidative initiated polymerization as illustrated in equations (1')-(4'). ##STR2## with respect to the electron transfer process, the initiator can function as an electron acceptor (reductive transfer) or as an electron donor (oxidative transfer) depending on the particular gegen ion employed as the coinitiator and, therefore, can initiate photoreductive or photooxidative polymerization.
Various compounds have been studied and disclosed as useful electron acceptors. For example, Toshihiro et al., Photog. Sci. Eng., 18, 25 (1974) and "Bull. Chem. Soc.," Japan, Vol. 48, p. 3737 (1975) teach the use of diazonium salts as electron acceptors. Diazosulfones were employed as electron acceptors by G. A. Delzenne, et al. in British Pat. No. 1,277,029 and in J. Photog. Sci., Vol. 22, p. 23 (1974). U.S. Pat. No. 4,257,915 to Eaton teaches the use of nitrohalomethanes as electron acceptors. Other compounds such as aryliodonium, sulfonium and phosphonium salts have been studied, for example, in U.S. Pat. Nos. 3,729,313 and 3,741,769, both to G. H. Smith.